Large numbers of people are killed or injured annually in automobile accidents wherein the driver and/or passengers are thrown forward so as to impact against solid surfaces within the vehicle. There has thus been considerable impetus toward development of passive restraint systems for use with these vehicles. One system which has been extensively investigated senses rapid deceleration of the vehicle such as that which occurs upon a primary impact between an automobile and, for example, another car. It thus initiates inflation of a bag between the interior surface of the car and the vehicle occupant prior to the occurrence of any secondary collision between the driver and/or passengers and the interior of the car. Inflation of the bag must therefore occur within milliseconds of the primary impact in order to restrain any occupants before they are injured due to secondary collisions against the solid surfaces within the vehicle.
Moreover, it is desirable to deflate the bag as soon as the impact of a crash is completed, so that the occupant is not trapped within the vehicle by an inflated bag. It is also desirable to deflate the bag rapidly so that, in case of accidental inflation, the restraint upon the person driving the automobile is sufficiently short that they do not lose control of the vehicle. In order to meet such criteria, specifications have been established whereby a bag should be inflated sufficiently to restrain an occupant in about 30-60 milliseconds after initiation, with substantial deflation occuring after about 100 milliseconds.
One of the problems with providing such a passive restraint system for protecting the driver of the automobile during a crash is how to arrange and position the device upon the steering column. For reasons of styling and consumer acceptance, as well as not interfering with the driver's movement or vision of the instrument panel, and so as not to degrade the vehicle's steering dynamics, it is desirable to arrange the crash restraint apparatus as conveniently as possible, and yet have it positioned so that it effectively accomplishes its intended protective function. Further, since an inflating device for such a crash restraint system must be capable of discharging a relatively large volume of gas in a very short period of time (e.g., 30-60 milliseconds), there are safety considerations not only in the deployment of the inflator within the automobile, but also with regard to handling, installing, replacing and repairing such inflating devices.
In addition, in the particular case of a driver's side, i.e., steering wheel, installation, the utilization of an inflator apparatus with a low weight is important for several reasons. First, the wheel assembly is in a cantilevered position at the end of the steering column. Therefore, excessive weight upon the wheel assembly can cause excessive column whip attributable to vertical accelerations due to road shocks (e.g., bumps or chuckholes) which can lead to degraded if not loss of driver control. Secondly, if for reasons such as styling or driver vision line clearance, the inflator is required to be located asymmetrically with respect to the steering column centerline, any excessive weight attributable to this device will create resistive wheel turning torque, thus degrading the rotational dynamics of the steering assembly and providing potential for loss of driver control or other unusual or undesirable vehicle handling "feel" under certain driving conditions.
The recent emphasis on weight reduction for the purpose of fuel conservation in motorized vehicles, has thus created a need and a demand for a lighter weight inflation system. This is of particular importance in a crash protection system for the driver wherein the inflator is mounted on the vehicle's steering wheel. The availability of a lighter weight inflator for installation at this location further enables a reduction to be made in the weight of the vehicle's steering wheel and the steering column on which the inflator is mounted, providing a concurrent improvement in the "steerability" of the automobile.
In this regard, some recently introduced inflator devices utilize aluminum casing materials. The use of lighter materials such as aluminum in the construction of automobile air bag inflators, however, creates certain difficulties in that techniques need to be developed for rapidly connecting components formed of this material together in such a way as to ensure the formation of a structural seal therebetween, even while the generator is pressurized during the inflation cycle.
As noted above, there are in the prior art various devices which cause a protective bag to inflate in front of an automobile driver or passenger to cushion the impact with the steering wheel, dashboard or other interior vehicle surface. Usually the device is activated by an inertial switch responsive to a primary crash impact. This inertial switch in turn causes an inflator apparatus to quickly inflate a collapsed bag into a protective position in front of the driver or passenger.
The inflating gas is generally supplied either from a source of compressed air or other compressed gas, such as shown in Chute, U.S. Pat. No. 3,411,808 and Wissing et al., U.S. Pat. No. 3,413,013, and a number of other patents in the crash restraint field. In several other prior art patents (e.g., U.S. Pat. Nos. 3,880,447 to Thorn et al.; 4,068,862 to Ishi et al.; 4,711,466 to Breed; and 4,547,342; 4,561,675 and 4,722,551 to Adams et al.), the bag is inflated by igniting a pyrotechnic propellant composition and directing the gaseous combustion products produced thereby directly into the bag.
The first technique discussed above for inflating an air bag requires a reservoir of gas stored at a very high pressure, which may be discharged into the bag as soon as an impact is sensed. In order to obtain a sufficient volume of gas for inflating a vehicle occupant restraint bag, however, a relatively large reservoir of gas, at pressures of 3000 psi or more is required. To open the gas reservoir in the very short time interval required for ensuring the safety of the vehicle occupants, explosive arrangements have been employed in the prior art for bursting a diaphragm or cutting through a structural portion of the reservoir. Such explosive arrangements have significant inherent safety problems, such as the production of shrapnel by the explosion, as well as the relatively high sound level reached within the passenger compartment due to the explosion. The psychological factor of having these explosives in each automobile also cannot be ignored.
The gas bottle (i.e., reservoir) technique for inflating an air bag also suffers from an additional disadvantage in that the gas pressure is highest at the commencement of bag deployment and decreases as a function of time as the gas in the storage reservoir is depleted. Moreover, the pressure/time history of such pressurized gas inflator devices is difficult if not impossible to control at reasonable cost and reliability. Further, the pressurized gas technique is undesirable since a minor leak can result in all of the gas being lost during the long period that the passenger restraint system must remain in the automobile prior to any crash.
In addition, the adiabatic cooling of the gas as it expands from a storage condition of elevated pressure to the nearly ambient pressure of the inflatable bag reduces the effective volume of the gas available for inflating the bag. This cooling effect thus requires the manufacturer of the device to provide a total gas storage volume significantly greater than if the gas was initially stored at an elevated temperature.
The second technique discussed above employs a pyrotechnic gas generator having a rapidly burning propellant composition stored therein for producing substantial volumes of hot gaseous products which are then directed into the inflatable bag. Some compositions are available which produce a sufficiently low temperature combustion gas such that the gas may be directed substantially directly into the bag without danger to the vehicle's occupants. Other systems produce a high temperature combustion product requiring means for cooling the gas before it is introduced into the bag.
Many forms of gas generators or inflators utilizing combustible solid fuel gas generating compositions for the inflation of crash protection, i.e., "air bag", restraint systems are known in the prior art. Commonly encountered features among generators utilized for this purpose include: (1) an outer metal housing, (2) a gas generant composition located within the housing, (3) means to ignite the gas generant responsive to a signal received from a sensor positioned at a location removed from the inflator, and (4) means to filter and to cool the gas, positioned between the propellant composition and a plurality of gas discharge orifices defined by the generator housing.
One such gas generator includes an annular combustion chamber which is bounded by a welded outer casing or housing structure. The combustion chamber encloses a rupturable container or cartridge that is hermetically sealed and which contains a solid gas generant in pelletized form, surrounded by an annular filter assembly. The device further includes a central ignition or initiator zone and a toroidal filter chamber adjoining and encircling the combustion chamber. An inner casing or housing structure is located in close surrounding and supporting relationship to the rupturable container, the inner casing being formed by a cylinder having uniformly spaced peripheral ports or orifices near one end. These orifices provide exit holes to facilitate the flow of gas from the combustion chamber.
Alternately, inflator devices are constructed comprising first and second structural components or shells specifically, a first diffuser shell and a second base shell. Both shells are forged and heat treated, after which they undergo machining to obtain a proper fit therebetween. The first structural component, i.e., the diffuser shell, comprises three integral concentric cylinders. These cylinders form the inner structural walls of the inflator and define chambers therein containing the solid gas generant, the ignition means, and the filter assembly. The cylinder walls further define exit openings or ports for the passage of the gases between adjacent chambers and subsequently out of the inflator and into the protective air bag.
The second structural component, i.e., the base shell, is equipped with an initiator device, i.e., an electrical squib, for igniting the main propellant charge. A flange is provided around the outer periphery of the base shell for attaching an air bag thereto. The base shell additionally comprises three concentric mating surfaces corresponding to the concentric cylinders of the diffuser shell. The three concentric cylinders of the diffuser shell are thus mated to corresponding concentric mating surfaces located upon the base shell by a process such as inertia welding.
As noted above, gas filtration systems are normally utilized with generators of the type described above, to cool the gas and to remove particulate products produced as a result of the combustion of the pyrotechnic material. Filters included in prior art gas generators of the type described above ordinarily comprise a series of zones or chambers containing layers of metal screen material having a variety of mesh sizes and/or one or more layers of an inert fiber. These filter components are typically separated from the central combustion chamber by thick support walls, which are required in this type of generator construction to withstand the elevated pressures produced during the ignition and combustion of the gas generant. A plurality of openings or ports are provided in these walls, through which the gas must pass in order to reach the filtration zone. Moreover, some sort of clip or pedestal arrangement within the filtration zone is normally required to maintain the plurality of screens, pads, etc. in proper position and alignment.
Gas generators must withstand enormous thermal and mechanical stresses for a short period during the gas generation process. Thus, inflators that have been and are currently being used with automobile air bag devices have been fabricated using heavy gauge steel for the casing and other structural housing components, with these components being joined together by, for example, screw threads, roll crimping or welding. More specifically, each of the gas generator units presently in commercial production is assembled and sealed with, for example, the use of some form of welding technique, such as inertia welding or electron beam welding.